Cross-post from r/linux: How do I gain shell access to any one of multiple computers (or even simultaneously) that aren't on my LAN?

here's the link to the original thread.

There's got to be some version of Logmein for Linux, in that a daemon constantly runs on the machine that keeps a connection open to a server, and that an admin can connect to that server and thus control any machine that contacts that server via the daemon... or am I making stuff up? Basically, I want to be able to push a command (not necessarily control the screen, just get a shell prompt) to a machine, regardless of whether it's on my local network or not. How can I do this? Thanks in advance.